Postactivation potentiation, fiber type, and twitch contraction time in human knee extensor muscles.

In small mammals, muscles with shorter twitch contraction times and a predominance of fast-twitch, type II fibers exhibit greater posttetanic twitch force potentiation than muscles with longer twitch contraction times and a predominance of slow-twitch, type I fibers. In humans, the correlation between potentiation and fiber-type distribution has not been found consistently. In the present study, postactivation potentiation (PAP) was induced in the knee extensors of 20 young men by a 10-s maximum voluntary isometric contraction (MVC). Maximal twitch contractions of the knee extensors were evoked before and after the MVC. A negative correlation (r = -0. 73, P < 0.001) was found between PAP and pre-MVC twitch time to peak torque (TPT). The four men with the highest (HPAP, 104 +/- 11%) and lowest (LPAP, 43 +/- 7%) PAP values (P < 0.0001) underwent needle biopsies of vastus lateralis. HPAP had a greater percentage of type II fibers (72 +/- 9 vs. 39 +/- 7%, P < 0.001) and shorter pre-MVC twitch TPT (61 +/- 12 vs. 86 +/- 7 ms, P < 0.05) than LPAP. These data indicate that, similar to the muscles of small mammals, human muscles with shorter twitch contraction times and a higher percentage of type II fibers exhibit greater PAP.     

Activation varies among the knee extensor muscles during a submaximal fatiguing contraction in the seated and supine postures.

Ten young men sustained an isometric contraction of the knee extensor muscles at 20% of the maximum voluntary contraction (MVC) torque on three separate occasions in a seated posture. Subjects performed an isometric knee extension contraction on a fourth occasion in a supine posture. The time to task failure for the seated posture was similar across sessions (291 +/- 84 s; P > 0.05), and the MVC torque was similarly reduced across sessions after the fatiguing contraction (42 +/- 12%). The rate of increase in electromyograph (EMG) activity (%MVC) and torque fluctuations during the fatiguing contractions were similar across sessions. However, the rate of increase in EMG differed among the knee extensor muscles: the rectus femoris began at a greater amplitude (31.5 +/- 11.0%) compared with the vastus lateralis and vastus medialis muscles (18.8 +/- 5.3%), but it ended at a similar value (45.4 +/- 3.1%). The time to task failure and increase in EMG activity were similar for the seated and supine tasks; however, the reduction in MVC torque was greater for the seated posture. These findings indicate that the time to task failure for the knee extensor muscles that have a common tendon insertion did not alter over repeat sessions as had been observed for the elbow flexor muscles (Hunter SK and Enoka RM. J Appl Physiol 94: 108-118, 2003).     

Force-velocity relations and fiber composition in human knee extensor muscles.

Standardized measurements of dynamic strength of the kneee extensor muscles were performed in 25 healthy male subjects (17-37 yr) by means of isokinetic contractions, i.e., knee extensions with constant angular velocities. Overall variation between double determinations of maximal torque throughout the 90 degrees arc of motion (0 degrees = fully extended leg) averaged 10% for the different constant velocities chosen. At any given angle of the knee the torque produced was higher for isometric than for dynamic contractions. Dynamic torque decreased gradually with increased speed of shortening. Peak dynamic torque was reached at knee angles in the range: 55-66 degrees, with a displacement toward smaller knee angles with higher angular velocities. Correlations were demonstrated between peak torque produced at the highest speed of muscle shortening and percent as well as relative area of fast twitch fibers in the contracting muscle. In addition muscles with a high percentage of fast twitch fibers had the highest maximal contraction speeds. These observations on intact human skeletal muscle are consistent with earlier findings in animal skeletal muscle preparations.     

Potentiation of knee extensor contraction by antagonist conditioning contraction at several intensities

The purpose of this study was to examine the effect of graded conditioning contractions of the antagonist knee flexor muscles on the output characteristics of knee extensor muscles in healthy humans. Eight male university students performed maximum isometric contractions of knee extensors, preceded by isometric conditioning contractions of the antagonist knee flexors. The developed force and electromyographic (EMG) amplitudes of the knee extensors after the conditioning contraction were measured and compared with those of simple knee extension without conditioning. The forces of the conditioning flexor contraction were set at three levels: low (20% of maximum voluntary contraction: MVC), moderate (60% of MVC), and high (100% of MVC). The EMG amplitudes of the vastus medialis, vastus lateralis, and rectus femoris muscle were recorded and the root mean square amplitudes were calculated. The strongest enhancement of the extension force was obtained by moderate intensity conditioning contraction (108.95+/-1.87% of simple knee extension), although high intensity conditioning also induced a significant increase (105.41+/-2.69%). Low intensity conditioning did not cause a significant enhancement of the contraction force (103.17+/-2.99%). Similarly, the EMG amplitudes were significantly increased by moderate and/or high conditioning. These results suggest that antagonist conditioning contraction of moderate intensities is sufficient and may be optimal to potentiate knee extensor contraction.     

Fiber number and type composition in extensor digitorum longus, soleus, and diaphragm muscles with aging in Fisher 344 rats

Histochemical (M-ATPase) fiber typing was done on extensor digitorum longus, (EDL), soleus (SOL), and diaphragm (DIA) muscles of barrier-reared Fisher 344 rats obtained at four different ages (3, 9, 28, and 30 months) from the colonies of the National Institute of Aging. In the EDL there are no differences in the percent of type I fibers among the four age groups. The percent of type IIa and IIb fibers also showed no difference between the 3 and 30 month age groups. There was no apparent trend for an increase or decrease in the percent of type IIa or IIb fibers between the four age groups. In both the SOL and DIA muscles the percent of type I fibers was greater in the aged than in the young groups. The percent of type IIa fibers was lower in the 30 month group than in the younger groups for both muscles. The percent of type IIb (DIA) and IIc (SOL) fibers did not change between groups. Total fiber number per cross section of muscle showed no change in the EDL over this age range or in the SOL after 9 months of age. These findings bring into question published results that imply that decreasing fiber number and preferential loss of type II (a and b) fibers are typical aging phenomena.     

Total power output generated during dynamic knee extensor exercise at different contraction frequencies.

A novel approach has been developed for the quantification of total mechanical power output produced by an isolated, well-defined muscle group during dynamic exercise in humans at different contraction frequencies. The calculation of total power output comprises the external power delivered to the ergometer (i.e., the external power output setting of the ergometer) and the "internal" power generated to overcome inertial and gravitational forces related to movement of the lower limb. Total power output was determined at contraction frequencies of 60 and 100 rpm. At 60 rpm, the internal power was 18+/- 1 W (range: 16-19 W) at external power outputs that ranged between 0 and 50 W. This was less (P<0.05) than the internal power of 33+/-2 W (27-38 W) at 100 rpm at 0-50 W. Moreover, at 100 rpm, internal power was lower (P<0.05) at the higher external power outputs. Pulmonary oxygen uptake was observed to be greater (P<0.05) at 100 than at 60 rpm at comparable total power outputs, suggesting that mechanical efficiency is lower at 100 rpm. Thus a method was developed that allowed accurate determination of the total power output during exercise generated by an isolated muscle group at different contraction frequencies.     

Differential adaptations during growth spurt and in young adult rat muscles

During the post-weaning growth and maturation period (25/90 days after birth), rat limb muscles are submitted to specific adaptations. Our aim was to characterize the mechanical properties of two muscles that are opposite in terms of fibre-type distribution, the soleus and the extensor digitorum longus (EDL) muscles of male Wistar rats. Results showed a fast-to-slow fibre-type transition in soleus while no modification in fibre-type distribution was observed in EDL. A growth-induced increase in muscle force was observed. Soleus underwent an increase in twitch kinetics, but EDL showed no modification. Resistance to fatigue was higher in 90-day-old soleus but not modified in the EDL. Surprisingly, analysis of maximal shortening velocity showed a decrease in both soleus and EDL. Finally, tension/extension curves indicated a growth-induced increase in series elastic stiffness in the two muscles. These results suggest that during this growth period, skeletal muscles are submitted to differential adaptations. Moreover, whereas adaptation of biomechanical properties observed can be explained partly by an adaptation of fibre profile in soleus, this is not the case for EDL. It is suggested that changes in muscle architecture, which are often disregarded, could explain some variations in mechanical properties, especially when muscles undergo an increase in both mass and length.     

Respiration during isometric contraction of the flexor or extensor muscles of the arm in humans

An increase in the instantaneous pulmonary flow and a decrease in the respiratory frequency has been found in man as a consequence of isometric contraction of either flexor or extensor muscles of the elbow. The delay of 200 to 900msec between the onset of the muscle contraction and the beginning of the flow increment suggests that the respiratory system increases its activity due to the proprioreceptor activation. As there is no significant difference in the results obtained during flexor or extensor contraction, the contribution of the two different receptor groups appears to be the same.     

Older adults are less steady during submaximal isometric contractions with the knee extensor muscles.

This study compared the steadiness of submaximal contractions with the knee extensor muscles in young and old adults. Twenty young and twenty old subjects underwent assessment of isometric maximum voluntary contraction (MVC), one-repetition maximum (1-RM) strength, and steadiness during isometric, concentric, and eccentric contractions with the knee extensor muscles. The old adults displayed 33% lower MVC force and a 41% lower 1-RM load. The coefficient of variation for force was significantly greater for the old adults during isometric contractions at 2, 5, and 10% of MVC but not at 50% MVC. The decline in steadiness at low forces experienced by the men was marginally greater than that experienced by the women. The steadiness of concentric and eccentric contractions was similar in young and old adults at 5, 10, and 50% of 1-RM load. Old subjects exhibited greater coactivation of an antagonist muscle compared with young subjects during the submaximal isometric and anisometric contractions. These results indicate that, whereas the ability to exert steady submaximal forces with the knee extensor muscles was reduced in old adults, fluctuations in knee joint angle during slow movements were similar for young and old adults.     

Assessment of human knee extensor muscles stress from in vivo physiological cross-sectional area and strength measurements.

The physiological cross-sectional areas (CSAp) of the vastus lateralis (VL), vastus intermedius (VI), vastus medialis (VM) and rectus femoris (RF) were obtained, in vivo, from the reconstructed muscle volumes, angles of pennation and distance between tendons of six healthy male volunteers by nuclear magnetic resonance imaging (MRI). In all subjects, the isometric maximum voluntary contraction strength (MVC) was measured at the optimum angle at which peak force occurred. The MVC developed at the ankle was 746.0 (SD 141.8) N and its tendon component (Ft), given by a mechanical advantage of 0.117 (SD 0.010), was 6.367 (SD 1.113) kN. To calculate the force acting along the fibres (Ff) of each muscle, Ft was divided by the cosine of the angle of pennation and multiplied for (CSAp.sigma CSAp-1), where sigma CSAp was the sum of CSAp of the four muscles. The resulting Ff values of VL, VI, VM and RF were: 1.452 (SD 0.531) kN, 1.997 (SD 0.187) kN, 1.914 (SD 0.827) kN, and 1.601 (SD 0.306) kN, respectively. The stress of each muscle was obtained by dividing these forces for the respective CSAp which was: 6.24 x 10(-3) (SD 2.54 x 10(-3)) m2 for VL, 8.35 x 10(-3) (SD 1.17 x 10(-3)) m2 for VI, 6.80 x 10(-3) (SD 2.66 x 10(-3)) m2 for VM and 6.62 x 10(-3) (SD 1.21 x 10(-3)) m2 for RF. The mean value of stress of VL, VI, VM and RF was 250 (SD 19) kN m-2; this value is in good agreement with data on animal muscle and those on human parallel-fibred muscle.     

Force fluctuations are modulated by alternate muscle activity of knee extensor synergists during low-level sustained contraction.

The study examined the hypothesis that altered synergistic activation of the knee extensors leads to cyclic modulation of the force fluctuations. To test this hypothesis, the force fluctuations were investigated during sustained knee extension at 2.5% of maximal voluntary contraction force for 60 min in 11 men. Surface electromyograms (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles. The SD of force and average EMG (AEMG) of each muscle were calculated for 30-s periods during alternate muscle activity. Power spectrum of force was calculated for the low- (< or =3 Hz), middle- (4-6 Hz), and high-frequency (8-12 Hz) components. Alternate muscle activity was observed between RF and the set of VL and VM muscles. The SD of force was not constant but variable due to the alternate muscle activity. The SD was significantly greater during high RF activity compared with high VL and VM activity (P < 0.05), and the correlation coefficient between the SD and AEMG was significantly greater in RF [0.736 (SD 0.095), P < 0.05] compared with VL and VM. Large changes were found in the high-frequency component. During high RF activity, the correlation coefficient between the SD and high-frequency component [0.832 (SD 0.087)] was significantly (P < 0.05) greater compared with other frequency components. It is suggested that modulations in knee extension force fluctuations are caused by the unique muscle activity in RF during the alternate muscle activity, which augments the high-frequency component of the fluctuations.     

Femoral artery inflow in relation to external and total work rate at different knee extensor contraction rates.

Whether limb blood flow is directly regulated to match the work rate, independent of the rate of contraction, remains elusive. This study therefore investigated the relationship between femoral arterial blood flow (FABF; Doppler ultrasound) and "external" (applied load) as well as "total" [external + "internal" (potential and kinetic energy changes of the moving lower leg)] work rate, during steady-state one-legged, dynamic, knee extensor exercise (1L-KEE) in the sitting position at different contraction rates. Ten subjects performed 1L-KEE at 30, 60, and 90 contractions/min (cpm) 1) at constant resistive loads of 0.2 and 0.5 kg inducing incremental external work rates (study I) and 2) at different relative resistive loads inducing constant external work rates of 9 and 18 W (study II). Moreover, 3) six subjects performed 1L-KEE at 60 and 100 cpm at incremental total work rates of 40, 50, 60, and 70 W (study III). In study I, FABF increased (P < 0.001) with increasing contraction frequency and external work rate, for each resistive load. In study II, FABF increased (P < 0.001) with increasing contraction frequency for each constant external work rate. Of major importance in study III, however, was that FABF, although increasing linearly with the total work rate, was not different (P = not significant) between contraction rates, at the total work rates of 40, 50, 60, and 70 W, respectively. Furthermore, FABF correlated linearly and positively with both the external and total work rate for each contraction frequency. In conclusion, the findings support the concept that leg blood flow during 1L-KEE in a normal knee extensor ergometer is matched directly in relation to the total work rate and metabolic activity, irrespective of the contraction frequency. The rate of contraction seems erroneously to influence the results only when it is related to the external work rate without taking into account the internal work component.     

Local blood circulation among knee extensor synergists in relation to alternate muscle activity during low-level sustained contraction.

The relation between local circulation and alternate muscle activity among knee extensor synergists was determined during low-level sustained knee extension at 2.5% of maximal voluntary contraction for 60 min in seven subjects. Blood volume of rectus femoris (RF) and vastus lateralis (VL) was assessed by using near-infrared spectroscopy. Surface electromyogram (EMG) was recorded from RF, VL, and vastus medialis (VM). Alternate muscle activity was observed between RF and either VL or VM. Cross-correlation analysis was used to investigate the relation between blood volume and integrated EMG (iEMG) sequences throughout the task. One negative peak in the cross-correlation function was seen between the iEMG and blood volume with time lag of 30-60 s, indicating that muscle activity increases (or decreases) with the decrease (or increase) in local circulation with the corresponding time lag. Two cases in the emergence of alternate muscle activities, i.e., an increase in the EMG of RF accompanied by a decline of EMG in VL (case I) and vice versa (case II) were further analyzed. The time lag between iEMG and blood volume was longer in case I than that in case II. These results were statistically significant in the RF but not in the VL. It is concluded that even during low-level sustained contraction, local circulation is modulated by the alternate muscle activity of knee extensor synergists, and a negative correlation between the muscle activity and blood volume sequences was found in only RF but not in VL.     

Flightin is essential for thick filament assembly and sarcomere stability in Drosophila flight muscles

Flightin is a multiply phosphorylated, 20-kD myofibrillar protein found in Drosophila indirect flight muscles (IFM). Previous work suggests that flightin plays an essential, as yet undefined, role in normal sarcomere structure and contractile activity. Here we show that flightin is associated with thick filaments where it is likely to interact with the myosin rod. We have created a null mutation for flightin, fln(0), that results in loss of flight ability but has no effect on fecundity or viability. Electron microscopy comparing pupa and adult fln(0) IFM shows that sarcomeres, and thick and thin filaments in pupal IFM, are 25-30% longer than in wild type. fln(0) fibers are abnormally wavy, but sarcomere and myotendon structure in pupa are otherwise normal. Within the first 5 h of adult life and beginning of contractile activity, IFM fibers become disrupted as thick filaments and sarcomeres are variably shortened, and myofibrils are ruptured at the myotendon junction. Unusual empty pockets and granular material interrupt the filament lattice of adult fln(0) sarcomeres. Site-specific cleavage of myosin heavy chain occurs during this period. That myosin is cleaved in the absence of flightin is consistent with the immunolocalization of flightin on the thick filament and biochemical and genetic evidence suggesting it is associated with the myosin rod. Our results indicate that flightin is required for the establishment of normal thick filament length during late pupal development and thick filament stability in adult after initiation of contractile activity.     

Enzymic and metabolic adaptations in the gastrocnemius, plantaris and soleus muscles of hypocaloric rats.

1. The effect of hypocaloric feeding (25% of normal food intake for 21 days) of rats on the enzymic and metabolic adaptations in the gastrocnemius, plantaris and soleus muscles was studied. 2. In control and hypocaloric rats the muscle relaxation rates at 100 Hz were 35.76 and 11.38% force loss/10 ms respectively. Control rats exhibited enhanced force of muscle contraction as the frequency of stimulation increased from 10 to 100 Hz, with maximum force being at 100 Hz. Hypocaloric rats exhibited a decrease in the increment of force being exerted at high frequencies, with maintenance of force at lower stimulatory frequencies. 3. In muscles of hypocaloric rats, there were significant decreases in the maximal activities of hexokinase (17.6-37.0%), 6-phosphofructokinase (22.7-34.2%), pyruvate kinase (21.2-36.0%), citrate synthase (34.1-41.5%), oxoglutarate dehydrogenase (29.4-52.4%) and 3-hydroxyacyl-CoA dehydrogenase (26.7-32.1%), whereas the activities of glycogen phosphorylase increased (23.8-43.4%) compared with control values. 4. In soleus-muscle strip preparations of hypocaloric rats, there were significant decreases in the rates of lactate production (28.1%) and glucose oxidation (32.6%) compared with control preparations. 5. Mitochondrial preparations from muscles of hypocaloric rats incubated with various substrates exhibited decreased rates of oxygen uptake compared with control preparations. 6. In muscles of hypocaloric rats (gastrocnemius and soleus), there were significant decreases in the concentrations of glycogen (P less than 0.001) and phosphocreatine (P less than 0.001) and increases in those of pyruvate (P less than 0.001), lactate (P less than 0.001) and ADP (P less than 0.001), whereas those of ATP and AMP remained unchanged. 7. Calculated [lactate]/[pyruvate] and [ATP]/[ADP] ratios exhibited significant increases (P less than 0.05) and decreases (P less than 0.05) in muscles of hypocaloric rats respectively. 8. The results are discussed in relation to the genesis of muscle dysfunction caused by malnutrition.     

Acute changes in motor unit discharge property after concentric versus eccentric contraction exercise in knee extensor

This study aimed to investigate the motor unit firing property immediately after concentric or eccentric contraction exercise. Eighteen healthy men performed repetitive maximal isokinetic knee extension exercises with only concentric or eccentric contraction until they exerted less than 80% of the baseline strength. Before and after the fatiguing exercise, high-density surface electromyography of the vastus lateralis was recorded during submaximal ramp-up isometric contraction and individual motor units were identified. Only motor units that could be tracked before and after exercise were analyzed. Muscle cross-sectional area of the vastus lateralis was measured using ultrasound, and electrically evoked torque was recorded before and after the exercise. Sixty-five and fifty-three motor units were analyzed before and after the concentric and eccentric contractions, respectively. The results showed that motor units with moderate to high recruitment thresholds significantly decreased recruitment thresholds under both conditions, and the motor unit discharge rates significantly increased after concentric contraction compared to eccentric contraction. A greater muscle cross-sectional area was observed with concentric contraction. The evoked torque was significantly decreased under both conditions, but no difference between the conditions. These results suggest that fatiguing exercise with concentric contraction contributes to greater neural input to muscles and metabolic responses than eccentric contraction.     

Free radical activity following contraction-induced injury to the extensor digitorum longus muscles of rats.

The purpose of the study was to investigate the role of free radicals in the injury induced by a protocol of repeated pliometric (lengthening) contractions to the extensor digitorum longus (EDL) muscle in situ in rats. Previous data have indicated that prior treatment with the antioxidant polyethylene glycol-superoxide dismutase reduced the damage that was apparent at 3 days following this type of exercise. Three hours and 3 days following the protocol, the magnitude of the semiquinone-derived free radical signal observed by electron spin resonance spectroscopy (ESR) was not different for exercised and non-exercised skeletal muscles. A reduction in the protein thiol content of muscle was evident at 3 h, and was still apparent at 3 days. Three hours after the protocol, the total muscle glutathione content and the percentage in the oxidized form were unchanged, but by 3 days the percentage of muscle glutathione present in the oxidized form was elevated. The susceptibility of muscle to lipid peroxidation in vitro was reduced 3 days after the pliometric contractions. These data indicate that oxidation of protein thiols and glutathione may be involved in the secondary damage following pliometric contractions, but provide no evidence that the species involved were derived from mitochondrial semiquinone radicals.